In signal transmission systems, received waves can be distorted due to reflected waves occurring over transmission channels. For example, in the case of a terrestrial wave TV broadcast, the radio waves direct from a transmitting tower are interfered with by the waves coming therefrom but getting detoured through reflection on buildings and mountains before arrival, as shown in FIG. 1. Such interference between direct and reflected waves has been known as “ghost” since the era of analog TV broadcasting. It is still a big problem affecting the receiving characteristics of digital TV broadcasts.
Illustratively, as shown in FIG. 2, comparing the spectrum in effect when there are no reflected waves (shown on the left) with the spectrum in effect when there are reflected waves (shown on the right) reveals the following: that whereas power density is held constant with regard to frequency when there are no reflected waves (on the left), the level of power density drops at a certain frequency where signal distortion is caused by reflected waves (on the right).
The waveform equalizer is used as a device to remove such distortion. There are diverse structures of the waveform equalizer. Generally, the waveform equalizer may be structured as a filter having a coefficient that constitutes the reverse characteristic of the transmission channel in use. FIG. 3 is explanatory of a waveform equalizer structured in such a manner.
As shown in FIG. 3, if the frequency characteristic of the channel coming from a broadcasting station is assumed to be H(f), then the waveform equalizer inside a receiving device receiving the broadcast signal is arranged to have a frequency characteristic of 1/H(f). This allows the waveform equalizer to output a interference-free signal to a demodulating/decoding section located downstream. That is, even if there exist reflected waves, it is possible to provide a spectrum without a dip as shown on the left of FIG. 2.
FIG. 4 shows a structure of a waveform equalizer. This waveform equalizer is made up of registers 111 through 11n each delaying the input signal by one clock pulse for output to the immediately subsequent stage, multipliers 120 through 12n each multiplying the input signal by one of filter coefficients (tap coefficients) C10 through C1n, and adders 131 through 13n adding up the products from the multipliers 120 through 12n.
In the waveform equalizer of FIG. 4, the registers 111 through 11n delay the input received signal by one clock pulse each, and the received signal thus delayed is multiplied by each of the filter coefficients (tap coefficients) C10 through C1n by each of the multipliers 120 through 12n. Then, the adders 131 through 13n add up the products from the multipliers 120 through 12n. The resulting sum of the products is output as an equalized signal.
As a result, the equalized signal is an interference-free signal. Incidentally, the filter coefficients by which the delayed received signal is multiplied by each of the multipliers 120 through 12n are acquired in keeping with impulse responses as shown in FIG. 5, to be discussed later.
Also, the waveform equalizers are roughly classified by the operating frequency into symbol rate equalizers and fractionally spaced equalizers.
Parenthetically, the symbol rate equalizer and fractionally spaced equalizer are discussed in detail in a book titled “Digital Communication,” written by John G. Proakis and translated by Koichi Sakaniwa et al into Japanese, published by Kagaku Gijutsu Shuppan, Inc. in November 1999 (ISBN: 978-4-87653-073-1 (4-87653-073-4)).
Comparing the two types of waveform equalizers reveals the following: that the symbol rate equalizer drives its filter using the symbol frequency of the transmitted signal, and that the fractionally spaced equalizer effects the driving using a frequency higher than the symbol frequency (usually by use of the frequency acquired by multiplying the symbol frequency by an integer multiple). For these reasons, there exist the following advantages and disadvantages regarding the two types of waveform equalizers.
First of all, where there are a sufficient number of taps, the fractionally spaced equalizer can perform equalization more accurately than the symbol rate equalizer. This is because the symbol rate equalizer does not satisfy sampling theorem and is thus theoretically incapable of reproducing the transmitted signal, whereas the fractionally spaced equalizer satisfies sampling theorem and is thus theoretically capable of reproducing the transmitted signal.
Second, given the same number of taps, the symbol rate equalizer can deal with longer delayed waves than the fractionally spaced equalizer. This is because the ability of a waveform equalizer to deal with longer delayed waves is determined by the length of the impulse response that can be expressed by the waveform equalizer in question. That is, as shown in FIG. 5, a fractionally spaced equalizer operating at an n-fold symbol rate needs n times as many taps as those for the symbol rate equalizer in order to express the impulse response of the same length.
FIG. 5 is a graphic representation showing the relationship between impulse responses and filter coefficients.
In the graph of FIG. 5, the vertical axis stands for impulse responses. The higher the value along the vertical axis, the larger the impulse response value. The horizontal axis denotes the time of which the direction is from left to right as seen in the graph.
In FIG. 5, the arrows extending toward the waveform of the impulse response are spaced at intervals of T/2. Therefore, the space T for two arrows corresponds to the period of the symbol rate equalizer (i.e., symbol period), and the space T/2 for one arrow corresponds to the period of the fractionally spaced equalizer. That means the fractionally spaced equalizer requires twice as many taps as those for the symbol rate equalizer in order to express the impulse response of the same length.
If it were possible to have a sufficiently large number of taps, then a highly accurate fractionally spaced equalizer might well be utilized. However, always providing a sufficiently large number of taps is not realistic in terms of costs and other considerations. Thus the designers of waveform equalizers need to design optimal circuits by taking the above-mentioned advantages and disadvantages into account.
In view of such trade-offs, Patent Document 1 below proposes techniques whereby a symbol rate equalizer and a fractionally spaced equalizer are incorporated in a receiving apparatus in such a manner that one of them is selected for use depending on the channel. According to the proposal, a selection can be made between the symbol rate equalizer and the fractionally spaced equalizer as needed.    Patent Document 1: Japanese Patent Laid-open No. Hei 3-244220